1. Field of the Invention
The present invention relates to a position detecting apparatus that uses a position detecting magnetic field having position dependency regarding strength to detect a position of a detected object, at least at a first time instant and a second time instant when a predetermined time has passed since the first time instant, and a body-insertable apparatus system.
2. Description of the Related Art
Recently, in the field of endoscope, a swallowable capsule endoscope has been proposed. The capsule endoscope is provided with an imaging function and a radio communication function. This capsule endoscope has a function of moving in a body cavity, for example, internal organs such as a stomach and a small intestine with peristalsis thereof, during a period after it is swallowed from a mouth of the subject for observation (examination) until it is naturally discharged from the subject, and of sequentially imaging intra-subject images.
While the endoscope is moving in the body cavity, image data imaged in the body by the capsule endoscope is sequentially transmitted to the outside by radio communications, and stored in a memory provided in an external device. If the subject carries a receiving device having the radio communication function and the memory function, the subject swallows the capsule endoscope and then can freely move until the endoscope is discharged. After the capsule endoscope is discharged, a doctor or a nurse can perform diagnosis by displaying the images of the internal organs based on the image data stored in the memory (see, for example, Japanese Patent Application Laid-open No. 2003-19111).
Further, in the conventional capsule endoscope system, one having a mechanism for detecting the position of the capsule endoscope in the body cavity has been proposed. For example, a magnetic field is generated, which has the position dependency regarding strength inside the subject into which the capsule endoscope is introduced, and the position of the capsule endoscope in the subject can be detected based on the magnetic field strength detected by a magnetic field sensor incorporated in the capsule endoscope. In such a capsule endoscope system, a configuration in which a predetermined coil is arranged outside the subject is adopted to generate the magnetic field, and by allowing predetermined electric current to flow to the coil, the magnetic field is generated inside the subject. Since it is difficult to detect the position of the capsule endoscope beforehand, the magnetic field to be generated needs to be generated so that the capsule endoscope has detectable strength in all areas where the capsule endoscope can be present inside the subject. Specifically, in the conventional capsule endoscope system, a magnetic field capable of detecting the capsule endoscope is generated in all the digestive organs from an oral cavity to an anus.
However, the conventional capsule endoscope system including a position detecting mechanism has a problem in that power consumption greatly increases. That is, to generate the magnetic field having the position dependency regarding the strength in the subject, large current needs to be continuously supplied to the coil over several to ten and odd hours, during which the capsule endoscope stays in the subject. Particularly, in the conventional capsule endoscope system, since the magnetic field having the strength capable of detecting the capsule endoscope is generated with respect to all the digestive organs in the subject, the power required for generating the magnetic field becomes huge, which is not appropriate from the standpoint of reducing the power consumption.
Further, the conventional capsule endoscope system including the position detecting mechanism has another problem in that the power consumption in at least the capsule endoscope increases. Specifically, in the conventional capsule endoscope system, position detection is performed at a constant time interval, and the power consumption increases by a portion of the magnetic field sensor incorporated in a capsule endoscope 2 and driving power of a transmitting mechanism for wirelessly transmitting a detection result of the magnetic field sensor.
Particularly, there is an assumption that it is preferable to form the capsule endoscope as small as possible, to reduce a burden on the subject. Therefore, a small battery or the like incorporated in the capsule endoscope is used, and there is generally a limitation on electric energy to be held. Accordingly, the influence due to an increase of power consumption in the capsule endoscope is larger than in the general electronic equipment, and suppression of increase in power consumption is quite important in the capsule endoscope system.